"If divorce is so unthinkable to evangelicals, then why do Christians have a higher divorce rate than atheists?"
I'd stack up the divorce rates of church-going Christians (or any other faith) against atheists anyday.
Sadly, the vast majority of people who report themselves as "Christian" are Christian in culture only, rarely if ever attending a church service.
I believe that Christians are more likely to marry. For those who live together, there's no statistic if they break up.
Churches have become social clubs for people who like to think they're better than the average Joe on the street. I've seen more selfish, heartless people in churches than I care to remember. Not my kind of place or my kind of people.